Wireless communication technology has evolved greatly over the recent years. Recent studies have shown that the wireless spectrum in U.S. is under-utilized, although most of the spectrum has been assigned to licensees, or primary users. Therefore, spectrum sharing has been proposed to alleviate the spectrum scarcity that prevents new wireless services being deployed. Cognitive radio is a promising technology that can allow secondary usage of spectrum without causing harmful interference to the primary systems and/or to any other active users. The secondary users, or cognitive radios, are required to perform spectrum (channel) sensing before accessing a channel.
In order to reliably detect an active signal on a channel, an individual radio needs to have very high sensitivity to offset the fading and shadowing conditions of a radio channel. Additionally, in order to achieve a desired spectrum sensing performance noise estimation operations are generally required to be performed by the cognitive radios. Estimation of noise levels is used to determine detection thresholds for the desired performance of the detection system. However, to reliably estimate noise levels for effective threshold selection, radios generally require a-priori knowledge of when a spectrum will be inactive. Unfortunately, cognitive radios usually do not have access to this a-priori information and, therefore, reliable noise estimation becomes an issue.